It has been commonly carried out to use a photosensitive resin composition as a printing plate and that has been a mainstream in various fields of letter press, lithography, relief printing gravure and flexographic printings. As to the photosensitive resin composition, that which is developable by an aqueous type solution has been strongly demanded in view of affection to the environment and of workability. As to the flexographic photosensitive resin composition developable by water, that comprising synthetic rubber and hydrophilic polymer has been put into the market. Moreover, in recent years, there has been developed a water-developable plate where resolution is further enhanced by the use of latex which is present in fine particles as a main component.
For preparing those printing plates, a method where the film of original picture is closely adhered to a photosensitive resin printing plate in vacuo and then active ray is irradiated through the original picture film (negative system) has been widely used. In the negative system, crosslinking reaction selectively proceeds in the exposed area to which the active ray is irradiated whereupon an image part is formed. After that, the non-crosslinked resin composition in the non-exposed area is removed by washing with a water-based developer to give an uneven relief.
Since such a printing plate material requires an original picture film, manufacturing time and cost necessary therefor are needed. In addition, chemical treatment is necessary for developing the original picture film and treatment of the waste developer is further necessary whereby disadvantages in terms of environmental hygiene is accompanied therewith.
As a result of progress in computers in recent years, there has been proposed a method (Computer to Plate, CTP system) where the information treated on a computer is directly outputted onto the photosensitive resin printing plate whereby a printing plate is obtained without the step for preparing the original picture film. In the CTP system, an infrared-sensitive layer which is non-transparent to the active ray is formed on the printing plate and, when said infrared-sensitive layer is evaporated by infrared ray laser, an image mask is formed. After that, the active ray is irradiated the same as in the negative system to form an image part and the non-exposed area is removed by washing with a water-based developer to give an uneven relief. Advantages in this system are that the above-mentioned manufacturing step for the original picture film is not necessary, that a treatment of the waste developer for the original picture film is not necessary whereby it is favorable in terms of environmental hygiene and moreover that a relief in a sharp structure is prepared.
In developing the photosensitive resin printing plate in the CTP system, the non-image area protected from light by an image mask during the active ray irradiation is removed by washing with a water-based developer. At that time, the image mask layer and the non-crosslinked resin composition are dissolved or dispersed in the developer. Now, when developing is repeated using the same water-based developer, there happen problems such as that the non-crosslinked composition (scum) dispersed in the developer sticks to a brush whereby the developing property lowers and that it sticks to the printing plate surface again whereby the quality of the plate surface is deteriorated. Similar problem is also present in a printing plate of a negative system. In view of the above, similar water-based developer to the negative system has been commonly used for preventing the problem caused by the non-crosslinked resin composition (scum). In the negative system, it has been said to be suitable to use water of pH 5.0 to 9.0 including the common household water or that in which an alkaline compound such as sodium hydroxide or sodium carbonate, a surfactant or a water-soluble organic solvent is made to contain. As to the surfactant, it has been said that sodium alkylnaphthalene sulfonate, sodium alkylbenzene sulfonate, etc. may be used (cf. Patent Document 1). Further, there has been known a water-based developer where water is compounded with an anionic surfactant and, upon necessity, with a pH adjusting agent, a washing promoter, etc. As to the specific examples of the anionic surfactant, there have been known a linear alkylbenzene sulfonate having an alkyl of average carbon number of 8 to 16, an α-olefinsulfonate having average carbon number of 10 to 20 and a saturated or unsaturated fatty acid salt having average carbon number of 10 to 22 (cf. Patent Document 2).
However, although the conventionally known water-based developers are effective to the disadvantage caused by the non-crosslinked resin composition (scum), the effect thereof to the image mask layer which is developed and removed together with the non-crosslinked composition is still insufficient. The image mask layer used in the CTP system contains much water-insoluble pigment such as carbon black having an infrared ray-absorbing ability. Accordingly, the image mask layer which is washed and removed in the developing step has poor water solubility and water dispersibility and there are problems such as that small pieces of the image mask layer stick to a brush whereby the developing property lowers and that they stick to the printing plate surface again whereby the quality of the plate surface is deteriorated.
In addition, although the conventionally known water-based developers are effective during the initial developing stage where the ratio by weight of the scum dissolved or dispersed in the developer to the developer is in low concentration, there happen the problems such as that the scum sticks to a brush whereby the developing property lowers and that it sticks to the printing plate surface again whereby the quality of the plate surface is deteriorated when the scum concentration rises to an extent of 10 parts by weight or higher as a result of developing in large quantities. Therefore, a developer where the scum concentration rises is to be discarded and the plate-making is to be started again using a fresh developer. However, it is not preferred in view of the load to the environment and also of the manufacturing cost that the discarding of the used developer and the supplementing of a new developer are frequently repeated.